U.S. patent number 4,196,963 [Application Number 05/910,407] was granted by the patent office on 1980-04-08 for method for eliminating li.sub.2 o out-diffusion in linbo.sub.3 and litao.sub.3 waveguide structures.
This patent grant is currently assigned to Hughes Aircraft Company. Invention is credited to Bor-Uei Chen, Antonio C. Pastor, Gregory L. Tangonan.
United States Patent |
4,196,963 |
Chen , et al. |
April 8, 1980 |
Method for eliminating Li.sub.2 O out-diffusion in LiNbO.sub.3 and
LiTaO.sub.3 waveguide structures
Abstract
A process for suppressing the out-diffusion of Li.sub.2 O from
LiNbO.sub.3 and LiTaO.sub.3 waveguide structures by exposing the
structures to a Li.sub.2 O-rich environment at sufficient vapor
pressure that Li.sub.2 O diffuses into the structure as a
compensation process and a solid-solid surface interaction occurs.
In one embodiment of the invention, the out-diffusion of Li.sub.2 O
from LiNbO.sub.3 and LiTaO.sub.3 crystals into which Ti has been
diffused is eliminated by annealing the structure in a high purity
powder of LiNbO.sub.3 or LiTaO.sub.3. In a second embodiment, the
Li.sub.2 O out-diffusion is partially suppressed by annealing the
structure in molten LiNO.sub.3. In a third embodiment of the
invention, a waveguide structure comprising a Li.sub.2 O-rich
guiding layer is formed by annealing LiNbO.sub.3 or LiTaO.sub.3
crystals in a high purity powder of LiNbO.sub.3 or LiTaO.sub.3,
which not only suppresses Li.sub.2 O out-diffusion but also
promotes Li.sub.2 O in-diffusion into the crystals.
Inventors: |
Chen; Bor-Uei (Northridge,
CA), Pastor; Antonio C. (Santa Monica, CA), Tangonan;
Gregory L. (Oxnard, CA) |
Assignee: |
Hughes Aircraft Company (Culver
City, CA)
|
Family
ID: |
25428736 |
Appl.
No.: |
05/910,407 |
Filed: |
May 30, 1978 |
Current U.S.
Class: |
385/130; 65/390;
65/400; 65/30.13; 65/117 |
Current CPC
Class: |
C01G
33/00 (20130101); C01G 35/00 (20130101); C30B
29/30 (20130101); C04B 35/495 (20130101); C30B
33/00 (20130101); C01P 2004/80 (20130101) |
Current International
Class: |
C30B
33/00 (20060101); C04B 35/495 (20060101); C01G
35/00 (20060101); C01G 33/00 (20060101); C03C
021/00 (); G02B 005/14 () |
Field of
Search: |
;65/3R,32,3E,117
;106/73.31,39.5 ;350/96.12 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
T R. Ranganath and S. Wang, "Suppression of Li.sub.2 O
Out-diffusion from Ti-diffused LiNbO.sub.3 Optical Waveguides",
Applied Physics Letters, vol. 30, No. 8, Apr. 15, 1977, pp.
376-379. .
S. Miyazawa, R. Guglielmi, and A. Carenco, "A Simple Technique for
Suppressing Li.sub.2 O Out-diffusion in Ti:LiNbO.sub.3 Optical
Waveguide", No. 11, Dec. 1, 1977, pp. 742-744. .
R. L. Holman, P. J. Cressman, and J. F. Revelli, "Chemical Control
of Optical Damage in Lithium Niobate", Applied Physics Letters,
32(5), Mar. 1, 1978, pp. 280-283. .
Bor-Uei Chen and Antonio C. Pastor, Elimination of Li.sub.2 O
Out-Diffusion Waveguide in LiNbO.sub.3 and LiTaO.sub.3 ", Applied
Physics Letters, vol. 30, No. 11, Jun. 1, 1977, pp.
570-571..
|
Primary Examiner: Fisher; Richard V.
Attorney, Agent or Firm: Lachman; Mary E. Bethurum; W. J.
MacAllister; W. H.
Claims
What is claimed is:
1. A process for treating lithium niobate (LiNbO.sub.3) and lithium
tantalate (LiTaO.sub.3) crystals which includes exposing a sample
of either LiNbO.sub.3 or LiTaO.sub.3 to a material selected from
the group of compounds consisting of LiNbO.sub.3, LiTaO.sub.3, and
LiNO.sub.3 and then annealing said sample for a period of time and
at a predetermined elevated temperature sufficient to produce a
uniform Li.sub.2 O vapor pressure and a Li.sub.2 O-rich environment
at the surface of said sample, whereby out-diffusion of Li.sub.2 O
molecules from said sample is suppressed.
2. A process as set forth in claim 1 wherein the exposure and
annealing of said sample comprises encapsulating said sample in a
high purity powder of a material selected from the group consisting
of LiNbO.sub.3 and LiTaO.sub.3 and then annealing said sample at
900.degree. C. in a flowing oxygen environment for approximately
one hour, whereby undesirable waveguide modes in said sample which
are associated with Li.sub.2 O out-diffusion therefrom, are
removed.
3. A process as set forth in claim 1 wherein the exposure and
annealing of said sample comprises encapsulating said sample in a
high purity powder of a material selected from the group consisting
of LiNbO.sub.3 and LiTaO.sub.3 and then annealing said sample at
930.degree. C. in a flowing oxygen environment for a period of time
between 15 and 20 hours whereby Li.sub.2 O out-diffusion from said
sample is suppressed and Li.sub.2 O in-diffusion is promoted, to
form a thin-film waveguide in which both TE and TM polarizations
are optimized.
4. A process as set forth in claim 1 wherein the exposure and
annealing of said sample comprises surrounding said sample with
molten LiNO.sub.3 and then annealing said sample at 250.degree. C.
in a flowing oxygen environment for 24 hours, whereby undesirable
waveguide modes in said sample which are associated with Li.sub.2 O
out-diffusion therefrom, are suppressed.
5. A method for forming an optical waveguide in LiNbO.sub.3 and
LiTaO.sub.3 crystals in which undesirable waveguide modes
associated with Li.sub.2 O out-diffusion from said crystals are
removed by elimination of Li.sub.2 O out-diffusion from said
crystals, which includes encapsulating a crystal of either
LiNbO.sub.3 or LiTaO.sub.3 in a high purity powder of a material
selected from the group consisting of LiNbO.sub.3 and LiTaO.sub.3
and then annealing said crystal at 900.degree. C. in a flowing
oxygen environment for approximately one hour.
6. A method for forming an optical waveguide in LiNbO.sub.3 and
LiTaO.sub.3 crystals in which undesirable waveguide modes
associated with Li.sub.2 O out-diffusion from said crystals are
minimized by suppression of Li.sub.2 O out-diffusion from said
crystals, which includes surrounding said crystal with molten
LiNO.sub.3 and then annealing said crystal at 250.degree. C. in a
flowing oxygen environment for 24 hours.
7. A method for forming a thin-film waveguide in LiNbO.sub.3 and
LiTaO.sub.3 crystals in which both TE and TM polarizations are
optimized for C-axis propagation by elimination of Li.sub.2 O
out-diffusion from said crystals and promotion of Li.sub.2 O
in-diffusion into said crystals, which includes encapsulating a
sample of either LiNbO.sub.3 or LiTaO.sub.3 in a high purity powder
of a material selected from the group consisting of LiNbO.sub.3 and
LiTaO.sub.3 and then annealing said sample at 930.degree. C. in a
flowing oxygen environment for a period of time between 15 and 20
hours.
8. An optical waveguide structure in LiNbO.sub.3 and LiTaO.sub.3
crystals in which Li.sub.2 O out-diffusion from said crystals is
suppressed, formed by:
(a) providing a sample of a material selected from the group
consisting of LiNbO.sub.3 and LiTaO.sub.3 crystals,
(b) exposing said sample to a material selected from the group of
compounds consisting of LiNbO.sub.3, LiTaO.sub.3 and LiNO.sub.3,
and
(c) annealing said sample for a period of time and at a
predetermined elevated temperature sufficient to produce a uniform
Li.sub.2 O vapor pressure and a Li.sub.2 O-rich environment at the
surface of said sample.
9. An optical waveguide structure in LiNbO.sub.3 and LiTaO.sub.3
crystals in which undesirable waveguide modes associated with
Li.sub.2 O out-diffusion from said crystals are removed, formed
by:
(a) providing a sample of a material selected from the group
consisting of LiNbO.sub.3 and LiTaO.sub.3 crystals,
(b) in-diffusing chosen transition metal ions into said sample,
(c) encapsulating the metal-diffused sample in a high purity powder
of a material selected from the group consisting of LiNbO.sub.3 and
LiTaO.sub.3, and
(d) annealing said sample at 900.degree. C. in a flowing oxygen
environment for approximately one hour.
10. An optical waveguide structure in LiNbO.sub.3 and LiTaO.sub.3
crystals in which undesirable waveguide modes associated with
Li.sub.2 O out-diffusion from said crystals are suppressed, formed
by:
(a) providing a sample of a material selected from the group
consisting of LiNbO.sub.3 and LiTaO.sub.3 crystals,
(b) in-diffusing transition metal ions into said sample,
(c) surrounding said metal-diffused sample with molten LiNO.sub.3,
and
(d) annealing said sample at 250.degree. C. in a flowing oxygen
environment for 24 hours.
11. A thin-film waveguide structure in LiNbO.sub.3 and LiTaO.sub.3
crystals in which both TE and TM polarizations are optimized for
C-axis propagation by elimination of Li.sub.2 O out-diffusion from
said crystals, formed by:
(a) providing a sample of a material selected from the group
consisting of LiNbO.sub.3 and LiTaO.sub.3 crystals,
(b) encapsulating said sample in a high purity powder of a material
selected from LiNbO.sub.3 and LiTaO.sub.3, and
(c) annealing said sample at 930.degree. C. in a flowing oxygen
environment for a period of time between 15 and 20 hours.
Description
FIELD OF THE INVENTION
This invention relates generally to processes for forming and
treating electro-optical materials and more particularly to a
process for preventing undesirable waveguide modes in lithium
niobate and lithium tantalate crystal substrates.
BACKGROUND OF THE INVENTION
In optical communication systems, messages are transmitted by
carrier waves of optical frequencies that are generated by sources
such as lasers or light-emitting diodes. There is much current
interest in such optical communication systems because they offer
several advantages over conventional communication systems, such as
a greatly increased number of channels of communication and the
ability to use other materials besides expensive copper cables for
transmitting messages. One such means for conducting or guiding
waves of optical frequencies from one point to another is called an
"optical waveguide." The operation of an optical waveguide is based
on the fact that when a medium which is transparent to light is
surrounded or otherwise bounded by another medium having a lower
refractive index, light introduced along the inner medium's axis is
totally reflected at the boundary with the surrounding medium, thus
producing a guiding effect.
Certain electro-optical materials are very attractive for this
application since they make it possible to achieve electrical
control and high-speed operation in light propagating structures.
The use of lithium niobate (LiNbO.sub.3) and lithium tantalate
(LiTaO.sub.3) crystals for such purposes is well-known in the art
and is disclosed, for example, in an article entitled "Integrated
Optics and New Wave Phenomenon in Optical Waveguides," P. K. Tien,
in Reviews of Modern Physics, Vol. 49, No. 2, (1977) pp. 361-420.
These latter materials have large electro-optic and acousto-optic
coefficients which are desirable to provide control of light
propagation in the optical waveguide. Many different types of
active channel waveguide devices using these materials have been
used in a variety of modulators and switches which are compatible
with single-mode optical fibers.
Various methods of forming high refractive index waveguides in
LiNbO.sub.3 and LiTaO.sub.3 have been used in the art. They
include: epitaxial growth by sputtering, epitaxial growth by
melting, lithium oxide (Li.sub.2 O) out-diffusion, and transition
metal in-diffusion. Epitaxial growth by sputtering often leads to
films with high losses and poor electro-optical properties. In
epitaxial growth by melting, the film thickness cannot be easily
controlled. The Li.sub.2 O out-diffusion process generates a film
which can support only TE polarization waves (polarization parallel
to the surface of the waveguide structure) propagating along the X
or Y axes.
The in-diffusion of a transition metal, such as titanium, nickel,
or vanadium, into LiNbO.sub.3 or LiTaO.sub.3 offers a promising
technique to produce planar as well as channel waveguide
structures. However, a serious problem arises with this latter
approach because at the high temperature required for metal
in-diffusion, loosely bound Li.sub.2 O diffuses out from the
surface of the crystal structure. As a result of this Li.sub.2 O
out-diffusion, a Li.sub.2 O-deficient planar waveguide layer is
formed in both the LiNbO.sub.3 and the LiTaO.sub.3 crystals in
addition to the waveguides formed by metal in-diffusion. The
out-diffusion waveguide can confine TE polarization waves
propagating along the X-axis on a Y-cut wafer (or the Y-axis on an
X-cut wafer) in an undesirable manner. (A Y-cut wafer is a wafer
cut perpendicular to the Y-axis of the crystal. For a more detailed
description of crystal cutting, refer to "Standards on
Piezoelectric Crystals, 1949," Proceedings of the Institute of
Radio Engineers, pages 1378-1395, Dec. 1949.) In a channel
waveguide device, a planar out-diffusion waveguide introduces
excessive cross-talk between guided modes from two adjacent
waveguides. Cross-talk presents particular difficulties when trying
to achieve compatibility between a fiber optic communications link
and optical channel waveguide switches/modulators. The planar index
increase in the C-axis caused by the out-diffusion of Li.sub.2 O
limits the implementation of the optical switches to TM modes only
(i.e., polarization perpendicular to the surface of the waveguide
structure). In addition, in an end-butt coupling configuration
between a single mode optical fiber and a channel waveguide, a
large portion of the optical energy goes to the unwanted
out-diffusion modes, which are readily excited by the optical fiber
input, and thus the coupling to the channel waveguide is
effectively diminished. It is the alleviation of these various
problems caused by Li.sub.2 O out-diffusion to which the present
invention is directed.
The cause of the out-diffusion of Li.sub.2 O from LiNbO.sub.3 and
LiTaO.sub.3 crystals is inherent in the particular structure of
these crystals. It is well known that LiNbO.sub.3 and LiTaO.sub.3
crystals can be grown in a slightly non-stoichiometric form,
(Li.sub.2 O).sub.v (M.sub.2 O.sub.5).sub.1-v where M may be Nb or
Ta and v ranges from 0.48 to 0.50. At the high temperature
(850.degree. C. to 1200.degree. C.) required for the in-diffusion
of transition metal ions in order to form a waveguide in
LiNbO.sub.3 and LiTaO.sub.3 crystals, the loosely bound Li.sub.2 O
diffuses out from the surface of the crystal. It is known
experimentally that for a small change of v in LiNbO.sub.3 and
LiTaO.sub.3, the ordinary refractive index remains unchanged while
the extraordinary refractive index (along the C-axis) increases
approximately linearly as v decreases. The reduction in the
Li.sub.2 O concentration at the surface of the crystal due to
out-diffusion thus forms a high-index layer which traps optical
beams in the resulting waveguide structure.
It has been reported by W. Phillips and J. M. Hammer in the Journal
of Electronic Materials, Vol. 4, p. 549, 1975 that Li enrichment at
the surface can be achieved by annealing the substrates in Li.sub.2
CO.sub.3 at 550.degree.-600.degree. C. for a period of about 60
hours. In experiments where diffused lithium-niobate-tantalate
waveguides were formed, all the LiTaO.sub.3 wafers were treated
with this Phillips et al process before polishing. After the
annealing treatment, the substrates became dark brown, were found
hard to polish, and were more susceptible to cracking. The brownish
color can be bleached out during the high-temperature metal
diffusion. Although a reduction in waveguide loss was noted, this
Li.sub.2 CO.sub.3 powder treatment failed to prevent the formation
of out-diffusion waveguides. We have also tried the Li.sub.2
CO.sub.3 annealing of Ti-diffused waveguides after diffusion, but
after 120 hours of annealing at 600.degree. C. in a flowing oxygen
atmosphere, the Li.sub.2 O out-diffusion waveguides persisted. The
present invention seeks to overcome the disadvantages of the prior
art processes for eliminating Li.sub.2 O out-diffusion and to more
effectively accomplish the suppression of Li.sub.2 O
out-diffusion.
SUMMARY OF THE INVENTION
The general purpose of this invention is to provide a novel means
to suppress the out-diffusion of Li.sub.2 O from LiNbO.sub.3 and
LiTaO.sub.3 waveguide structures and to eliminate the consequent,
undesirable waveguide modes produced thereby.
In order to accomplish this purpose, we have discovered and
developed, among other things, novel processes and devices in which
the out-diffusion of Li.sub.2 O from LiNbO.sub.3 and LiTaO.sub.3
waveguides is suppressed by exposing a sample of either LiNbO.sub.3
or LiTaO.sub.3 to either LiNbO.sub.3, LiTaO.sub.3 or LiNO.sub.3 in
chosen forms and then annealing the sample for a period of time and
at a temperature sufficient to produce a uniform Li.sub.2 O vapor
pressure and a Li.sub.2 O-rich environment at the surface of the
sample. More specifically, the present invention provides, in one
embodiment thereof, a process for forming a thin-film waveguide in
LiNbO.sub.3 and LiTaO.sub.3 crystals, comprising a Li.sub.2 O-rich
guiding layer, in which both TE and TM polarizations are optimized
by suppression of Li.sub.2 O out-diffusion and promotion of
Li.sub.2 O in-diffusion. This process includes annealing the sample
of LiNbO.sub.3 or LiTaO.sub.3 in a high purity powder of
LiNbO.sub.3 or LiTaO.sub.3. The present invention further provides,
in two embodiments thereof, two separate processes respectively by
which the out-diffusion of Li.sub.2 O from LiNbO.sub.3 and
LiTaO.sub.3 waveguides is suppressed. In one embodiment, the sample
of LiNbO.sub.3 or LiTaO.sub.3 is annealed in molten LiNO.sub.3 and
Li.sub.2 O out-diffusion is suppressed, while in another process
embodiment, the sample is annealed in a high purity powder of
LiNbO.sub.3 or LiTaO.sub.3 and Li.sub.2 O out-diffusion is
eliminated.
The present invention is based on the discovery that Li.sub.2 O can
be satisfactorily diffused into LiNbO.sub.3 and LiTaO.sub.3
crystals for optical fabrication purposes, which is believed to be
a hitherto unknown fact. The mechanism of this Li.sub.2 O
compensation process is believed to be as follows. At high
temperatures, the loosely bound Li.sub.2 O molecules tend to escape
from the surface of the LiNbO.sub.3 or LiTaO.sub.3 crystal
structure, as previously discussed. In accordance with the present
invention, the wafer is exposed to a Li.sub.2 O-rich environment
such that out-diffusion of Li.sub.2 O from the wafer is suppressed
and the compensation process of diffusing Li.sub.2 O back into the
wafer becomes thermodynamically favorable. In addition, under these
conditions, favorable solid-solid surface reactions occur. The
source of the Li.sub.2 O-rich environment is a high purity powder
of LiNbO.sub.3 or LiTaO.sub.3 for two embodiments of the invention
and is molten LiNO.sub.3 for a third embodiment of the
invention.
The suppression of the Li.sub.2 O out-diffusion from LiNbO.sub.3
and LiTaO.sub.3 waveguide structures in accordance with the present
invention prevents the formation of unwanted waveguide modes and
the associated problem of excessive cross-talk between guided
modes. Thus, it facilitates waveguide switching and modulation.
Further, our invention enables efficient end-butt coupling between
a single mode optical fiber and a channel waveguide to be
achieved.
Accordingly, it is an object of the present invention to provide
new and improved processes for suppressing the out-diffusion of
Li.sub.2 O from LiNbO.sub.3 and LiTaO.sub.3 waveguide
structures.
It is a further object to provide processes for eliminating
undesirable out-diffusion waveguide modes in LiNbO.sub.3 and
LiTaO.sub.3 waveguide structures.
Another object of the invention is to provide a process for forming
thin-film waveguides in LiNbO.sub.3 and LiTaO.sub.3 crystals.
A further object is to provide thin-film waveguides in LiNbO.sub.3
and LiTaO.sub.3 crystals in which both TE and TM polarizations are
optimized for C-axis propagation.
Still another object is to provide LiNbO.sub.3 and LiTaO.sub.3
waveguide structures in which undesirable out-diffusion waveguide
modes have been eliminated.
These and other objects of the invention will become more readily
apparent in the following description of the accompanying drawings
and of the examples.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates in schematic cross-section the formation of a
prior art waveguide structure in which Li.sub.2 O out-diffusion has
occurred.
FIG. 2 presents a flowchart illustrating some of the major steps in
the process sequence for three embodiments of the invention.
DETAILED DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates the formation of a prior art waveguide
structure. In FIG. 1a there is shown a wafer 2 of LiNbO.sub.3 or
LiTaO.sub.3 which is the substrate starting material. A thin layer
4 of titanium (Ti) metal is deposited on the surface of substrate 2
as shown in FIG. 1b, using well-known electron beam evaporation
procedures, as discussed by K. L. Chopra, "Thin Film Phenomena,"
Chapter 2, McGraw-Hill Book Company, New York, 1969. The structure
of FIG. 1b is then heated at 900.degree. C. for six hours. During
this heating procedure, Ti metal from layer 4 diffuses into the
substrate 2 and, at the same time, Li.sub.2 O diffuses out from the
surface of the substrate 2. Thus, the structure of FIG. 1c results,
in which the substrate 2 contains a layer 6 in which Ti
in-diffusion has occurred, with layer 4 becoming embedded in layer
6, and in which Li.sub.2 O out-diffusion has occurred, and a layer
8 in which further Li.sub.2 O out-diffusion has occurred. It is the
out-diffusion of Li.sub.2 O from layers 6 and 8 which creates the
difficulties in prior art structures as previously discussed.
In FIG. 2, there is shown a flowchart for some of the major steps
in the process sequence for three embodiments of the invention. The
first step, which is common to all three embodiments, is to provide
a sample of LiNbO.sub.3 or LiTaO.sub.3 crystals. For example, in
the case of LiNbO.sub.3, a Y-cut crystal is provided in the form of
a wafer one inch by one inch. Next, following branch I shown in
FIG. 2, in the practice of one process embodiment of the invention,
the sample is packed in a high purity (99.99%) powder of
LiNbO.sub.3 or LiTaO.sub.3 and then annealed at 930.degree. C. for
15 to 20 hours in flowing oxygen atmosphere. The waveguide
structure thus produced by Li.sub.2 O in-diffusion guides most of
the laser light in a surface layer of the wafer that is about 15
microns in depth. In addition, both the TE and TM polarizations of
this waveguide are optimized for C-axis propagation.
Turning now to branch II shown in FIG. 2, to demonstrate two
additional process embodiments of the invention, the sample of
LiNbO.sub.3 or LiTaO.sub.3 is subjected to in-diffusion of ions of
a transition metal, such as titanium, vanadium, or nickel. This is
accomplished, for example, by electron-beam evaporating a layer of
Ti, 200 angstroms thick onto the surface of the LiNbO.sub.3 sample
and diffusing Ti ions therefrom into the underlying substrate by
heating the Ti coated substrate at 900.degree. C. for six hours.
Then, following branch IIA shown in FIG. 2, the sample is placed in
molten LiNO.sub.3 and annealed at 250.degree. C. for 24 hours. As a
result of the Li.sub.2 O compensation the prism coupling of the
He-Ne laser light into the Li.sub.2 O out-diffused region is
reduced by more than 10 times.
Turning now to branch IIB, to demonstrate a third process
embodiment, after the sample of LiNbO.sub.3 or LiTaO.sub.3 has been
in-diffused with Ti as indicated above, the sample is packed in a
high purity powder of LiNbO.sub.3 and annealed at 900.degree. C. in
a flowing oxygen atmosphere from 1 to 4 hours. The resulting
waveguide structure showed complete suppression of waveguide modes
due to Li.sub.2 O out-diffusion, while the single mode of the Ti
in-diffusion waveguide was unaffected. Further details of this
embodiment are described in Example 3 herein, as well as in an
article entitled "Elimination of Li.sub.2 O Out-Diffusion Waveguide
in LiNbO.sub.3 and LiTaO.sub.3 ", Bor-Uei Chen and Antonio C.
Pastor, in Applied Physics Letters, Vol. 30, No. 11 (1 June 1977),
pp. 570-571. It has further been discovered that this encapsulation
treatment of the sample with LiNbO.sub.3 powder can be carried out
either before or simultaneously with the metal in-diffusion
process, as well as subsequent to metal in-diffusion as shown in
FIG. 2, branch IIB. In each alternative process embodiment,
suppression of the out-diffusion waveguide is achieved.
In addition, it has been found that the extent to which the
out-diffusion waveguide modes are suppressed is related to the
length of the annealing time and the annealing temperature. Eighty
percent of the Li.sub.2 O in-diffusion process, as indicated by
changes in refractive index, occurred within less than 30 minutes
of annealing and the asymptotic value of the change in the
refractive index was approached after 2.5 hours. The fast reaction
rate indicates that the LiNbO.sub.3 powder treatment is effected
both by the diffusion process and by solid-solid surface
interaction. This conclusion is further supported by the fact that
the desired change in the refractive index was substantially
reduced when the sample was wrapped in platinum foil during
annealing in the LiNbO.sub.3 powder, in order to avoid physical
contact with the LiNbO.sub.3 powder.
The details of each process are more completely described in the
following examples, which are presented in order for branch I,
branches II and IIA combined, and branches II and IIB combined, of
FIG. 2.
EXAMPLE 1
This example illustrates the process for forming a thin film
waveguide in LiNbO.sub.3 crystals in which both TE and TM
polarizations are optimized by suppression of Li.sub.2 O
out-diffusion and promotion of Li.sub.2 O in-diffusion into the
crystal. This example presents more details of the process defined
in FIG. 2, branch I.
A Y-cut LiNbO.sub.3 crystal was provided as the substrate starting
material. Then, the sample was completely surrounded by a high
purity powder of LiNbO.sub.3 contained in a crucible made of an
inert material such as platinum and annealed at 930.degree. C. for
15 to 20 hours. Using end-firing equipment as described by E.
Garmire, H. Stoll, and A. Yariv, "Optical Waveguiding in
Proton-Implanted GaAs," in Applied Physics Letters, Vol. 21, No. 3,
Aug. 1, 1972, the results indicated that the incident laser light
was guided in a surface layer of the substrate which extended to a
depth of about 15 microns. The surface waveguide can support both
TE and TM polarization waves propagating in the crystalline
C-axis.
EXAMPLE 2
This example illustrates one process embodiment for forming an
optical waveguide in LiNbO.sub.3 crystals in which undesirable
waveguide modes associated with Li.sub.2 O out-diffusion from the
crystal are removed. This example presents more detail of the
process defined in FIG. 2, branches II and IIA.
A Y-cut LiNbO.sub.3 crystal was provided as the substrate starting
material. Then, Ti metal was evaporated onto the surface of the
substrate by well-known electron-beam sputtering techniques, to a
thickness of 200 A and this composite structure was heated at an
elevated temperature of 900.degree. C. for six hours, thereby
producing Ti in-diffusion into the crystal. The resulting waveguide
structure showed a single waveguide mode due to the Ti in-diffusion
and four closely spaced waveguide modes due to Li.sub.2 O
out-diffusion. The excitation and monitoring of waveguide modes was
carried out using two prism couplers in the manner described by P.
K. Tien, R. Ulrich, and R. J. Martin, "Modes of Propagating Light
Waves in Thin-Deposited Semiconductor Films," in Applied Physics
Letters, Vol. 14, p. 291 (1969). The intensity of the out-diffusion
modes (under optimized coupling conditions) was equal to that of
the in-diffused modes prior to treatment. The sample was then
placed in molten LiNO.sub.3 in a high-temperature furnace at
250.degree. C. for 24 hours and the mode structure restudied.
Subsequent to the treatment, the waveguide modes due to
out-diffusion persisted but their intensity was reduced by more
than 10 times their original value. This indicates that the
excitation of these modes was reduced by the compensation for the
loss of Li.sub.2 O by the LiNO.sub.3 treatment.
EXAMPLE 3
This example illustrates another process embodiment for forming an
optical waveguide in LiNbO.sub.3 crystals in which undesirable
waveguide modes associated with Li.sub.2 O out-diffusion from the
crystal are removed. This example presents more detail of the
process defined in FIG. 2, branches II and IIB.
A Y-cut LiNbO.sub.3 crystal was provided as the substrate starting
material. Then Ti metal was evaporated onto the surface of the
substrate by well-known electron-beam sputtering techniques, to a
thickness of 200 A and diffused therein at 900.degree. C. for six
hours. The diffusion was carried out in a high-temperature furnace
in a flowing oxygen atmosphere, using a thermocouple to monitor the
temperature. The resulting waveguide structure showed a single
waveguide mode due to Ti in-diffusion and two closely spaced
waveguide modes due to Li.sub.2 O out-diffusion for TE polarization
light propagating along the direction of the X-axis. The excitation
and monitoring of waveguide modes was carried out using both prism
couplers as described by P. K. Tien, R. Ulrich, R. J. Martin, in
Applied Physics Letters, Vol. 14, p. 291 (1969) and end-firing
equipment as described by E. Garmire, H. Stoll, and A. Yariv,
"Optical Waveguiding in Proton-Implanted GaAs," in Applied Physics
Letters, Vol. 21, No. 3, Aug. 1, 1972. The measurement of the
optical intensity profile at the waveguide exit end indicated
waveguide diffusion depths of .about.4 .mu.m for the Ti
in-diffusion guide and .about.15 .mu.m for the Li.sub.2 O
out-diffusion guide. The sample was then packed in high-purity
LiNbO.sub.3 powder and fired at 900.degree. C. in an oxygen
atmosphere for four hours. After this powder treatment, waveguide
modes due to Li.sub.2 O out-diffusion were completely suppressed,
while the single mode of Ti in-diffusion waveguide was not
affected. No substrate coloration or surface deterioration was
evident. The minimum back-diffusion time for complete suppression
of Li.sub.2 O out-diffusion modes was about 1 hour at 900.degree.
C. when fresh LiNbO.sub.3 powder was used. The LiNbO.sub.3 powder
was prepared by heating at 900.degree. C. a stoichiometric mixture
of high-purity Li.sub.2 CO.sub.3 (99.99%) and optical-grade
Nb.sub.2 O.sub.5, consisting of 79% by weight of Nb.sub.2 O.sub.5
and 21% by weight of Li.sub.2 CO.sub.3. During the heating process,
the weight of the mixture was continuously monitored to ensure a
complete reaction as evidenced by the weight loss due to the
release of CO.sub.2 gas.
In the process described in Example 3 above, the sample was treated
with LiNbO.sub.3 powder after the Ti had been diffused into the
LiNbO.sub.3 substrate. The LiNbO.sub.3 powder treatment may
optionally be performed either before or simultaneously with the Ti
in-diffusion process. In the former case, the LiNbO.sub.3 wafer was
pretreated with LiNbO.sub.3 powder at 900.degree. C. for 16 hours,
and no out-diffusion waveguide mode was observed after a subsequent
Ti metal in-diffusion process. In the latter case, the Ti oxidation
and in-diffusion processes were performed with the LiNbO.sub.3
substrated packed in LiNbO.sub.3 powder; again the Li.sub.2 O
out-diffusion process was suppressed.
In addition, the LiNbO.sub.3 powder treatment described in Example
3 above has been performed on Ti in-diffused LiTaO.sub.3 waveguides
and the same results were obtained as for the LiNbO.sub.3
waveguides described herein.
While the invention has been particularly described with respect to
the preferred embodiments thereof, it will be recognized by those
skilled in the art that certain modifications in form and detail
may be made without departing from the spirit and scope of the
invention. In particular, the scope of the invention is intended to
include any combination of LiNbO.sub.3 crystals and LiTaO.sub.3
crystals with LiNbO.sub.3, LiTaO.sub.3, and LiNO.sub.3 materials
under conditions which will provide the Li.sub.2 O-rich environment
at sufficient vapor pressure to allow the Li.sub.2 O in-diffusion
compensation process and the solid-solid surface interaction to
occur.
* * * * *